The present invention relates to a substrate for a semiconductor device, such as an SOI substrate and a substrate for an FET which has an MFS structure (metallic film/ferroelectric film/semiconductor layer structure), which is suitable for crystal growth of a semiconductor layer or a fereroelectric layer on a silicon substrate through an insulation layer, and to a method of manufacturing such a substrate. More particularly, the present invention relates to a substrate for a semiconductor device in which a crystalline insulation layer may be grown through an insulation layer which has an excellent insulation characteristic on a silicon substrate for the purpose of growing a semiconducting crystal layer, a ferroelectric crystal layer, etc., and to a method of manufacturing such a substrate.
In relation to an SOI substrate for growing a semiconducting crystal layer on an insulation layer, for instance, among known methods are a method which requires to bond two silicon substrate seating oxide films with each other and polish one of the two substrates to thereby leave a thin semiconductor layer. And another method requires to inject oxygen or the like at a surface of a silicon substrate to a constant depth and to anneal so that an insulation layer is buried in a semiconductor substrate.
Meanwhile, in a semiconductor memory device using a ferroelectric layer, the ferroelectric layer is formed on a semiconductor layer or on a surface of an electrode metal such as platinum through an insulation film. In an MFS structure in which a ferroelectric layer is formed on a semiconductor layer, an oxide film is formed between the ferroelectric layer and the semiconductor layer, thereby degrading a crystal quality or morphology, or an interface state density between the ferroelectric layer and the semiconductor layer becomes large. Where the ferroelectric layer is formed on an insulation film, it is not possible to grow a ferroelectric layer which has an excellent crystal quality on the insulation film which is amorphous.
As described above, during fabrication of a semiconductor device, while it is necessary to epitaxially grow a semiconductor layer or a crystalline ferroelectric layer on a semiconductor substrate through an insulation layer in some cases, since the insulation layer is amorphous, it is not possible to grow a crystalline layer directly on a surface of the insulation layer.
Further, in the method which requires to polish one of bonded silicon layers for thinning the one silicon layer, it is extremely difficult to polish the silicon layer into an uniformly thin layer and polishing is laborious, and therefore, a quality crystalline surface is hardly obtained. In the method which requires to inject oxygen at a surface layer portion of a semiconductor substrate, on the other hand, ion bombardment greatly degrades a surface of a semiconductor layer, and therefore, a high quality crystalline surface is hardly obtained, either. As a result, a crystal layer which is formed on such a deteriorated crystalline surface as well has a deteriorated crystal quality.
On the other hand, the inventors of the present invention invented a method of epitaxially growing an YSZ thin film on a silicon substrate and presented the method to Shingaku Gihou (ED96-42, CPM96-27, May 1996). This method makes it possible to obtain a crystalline insulation layer on a silicon substrate and epitaxially grow a semiconductor layer or a ferroelectric layer on a surface of the YSZ. However, since an YSZ thin film which is formed on a silicon substrate is a crystalline metallic oxide film and hence migrates ions, electric insulation of the YSZ thin film is inferior to that of a silicon oxide film or a silicon nitride film, and therefore, the YSZ thin film slightly degrades electric characteristics.
The present invention has been made to solve such problems. Accordingly, an object of the present invention is to provide a substrate for a semiconductor device which is suitable to grow a crystal layer, such as a semiconductor layer and a ferroelectric layer, on other semiconductor layer through an insulation layer during fabrication of a semiconductor device and which sufficiently improves electric insulation against a silicon substrate which serves as a base.
Other object of the present invention is to provide a method of forming a crystalline insulation layer on a semiconductor layer through an insulation silicon compound.
A substrate for a semiconductor device according to the present invention comprises a crytalline silicon substrate, an insulation silicon compound layer which is formed on the silicon substrate, and a crystalline insulation layer which is epitaxially grown on the insulation silicon compound layer.
As herein termed, a xe2x80x9csubstrate for a semiconductor devicexe2x80x9d refers to a base for growing crystalline semiconductor layers and ferroelectric layers one atop the other, but not a complete base for a semiconductor device. In this context, a xe2x80x9csubstrate for a semiconductor devicexe2x80x9d covers a structure that an insulation silicon compound layer or a ferroelectric layer is formed in a portion of a semiconductor device, or on a stacked semiconductor layer, etc.
Where the crystalline insulation layer is formed by at least one species which is selected from a group of YSZ (yttria stabilized zirconia), Al2O3 (sapphire), CeO2 (ceria), MgO (magnesia) and ZrO2 (zirconia) and the insulation silicon compound layer is formed by at least one of silicon oxide, silicon nitride and silicon nitride oxide, a substrate for a semiconductor device which is particularly excellent in insulation and crystalline characteristics is obtained.
A method of manufacturing a substrate for a semiconductor device according to the present invention is comprising the steps of:
growing a crystalline insulation layer on a silicon substrate by sputtering a metal which forms said crystalline insulation layer from a target, and chemically combining with reactive gas around said silicon substrate; and
forming an insulation silicon compound layer by applying a voltage to said silicon substrate so that ions of said reactive gas around said substrate are attracted to a surface of said silicon substrate and chemically combined with silicon.
More specifically, the silicon substrate and the target are disposed facing each other within a reactive sputtering apparatus, the reactive gas is supplied into the apparatus in such a manner that there is a larger amount of the reactive gas around the substrate than around the target, and inert gas which is supplied into the apparatus is discharged and the crystal layer of the crystalline insulation substance is grown, whereby the substrate for a semiconductor device is obtained. Even more specifically, the target may be a composite target or an alloy target of zirconium (Zr) and yttrium (Y), the reactive gas may be oxygen, the crystalline insulation layer may be YSZ, and the crystalline insulation substance may be silicon oxide.
When Ce, Al, Mg or Zr is used as the target, it is possible to grow CeO2, Al2O3, MgO or ZrO2, respectively, as the crystalline insulation layer.
The reactive gas may be supplied into the sputtering apparatus with the target covered with a cover which has an opening at a portion of the target which is faced with the silicon substrate. This simple structure allows to deposit the layer as it is in the metal mode on the substrate without chemically combining the target and to epitaxially grow a compound of the metal of the target and the reactive gas on the substrate.